HTTP Accept header

Just a quick question on HTTP accept header. According to
spec, Accept header can include extensions in the format of custom tokens:

accept-extension = ";" token [ "=" ( token | quoted-string ) ]

What is the best way to get these tokens? Currently HttpRequestHeaders.Accept is defined as System.Net.Http.Headers.HttpHeaderValueCollection<MediaTypeWithQualityHeaderValue> which seems to ignore these custom extensions.

Do I have to go to to request.Headers["Accept"] and parse the value or there is alternative means (or the support will be up-coming)?